The interferons (IFNs) are a family of multifunctional cytokines that are part of the human body's defenses against infections and some cancers. IFNs act primarily by inducing the synthesis of effector proteins. Although IFNs are used in the treatment of various human cancers, including hairy cell leukemia, chronic myelogenous leukemia, and Kaposi's sarcoma, the mechanisms by which they act as antiproliferative agents are poorly understood. Previously, we characterized a 52-kDa nuclear phosphoprotein (p202) whose level rises 20-fold in cells following IFN treatment. Constitutive expression of p2O2 in transfected cells inhibits growth. p2O2 has been found to bind to the retinoblastoma tumor suppressor protein (pRb). This finding indicates a potential role for p2O2 in regulating the cell cycle. This notion is further supported by tide observations that p2O2 associated with the transcription factor E2F (E2F-1/DP-1) in vitro and in vivo and inhibited E2F-mediated transcription in transient transfection assays. Thus, p2O2 might mediate the growth-inhibitory activities of the IFNs by inhibiting E2F activity. The major goal of the proposed project is to determine how p2O2 mediates the growth-inhibitory activities of the IFNs. We hypothesize that p2O2 inhibits cell growth by inhibiting E2F activity. To test our hypothesis, we propose three specific aims: (1) To examine whether constitutive overexpression of p2O2 arrests cells in a particular phase of the cell cycle. For this purpose, we have generated murine AKR-2B cell lines in which the expression of p2O2 can be selectively induced by removal of tetracycline. (2) To determine how p2O2 inhibits E2F activity. To do this, we propose to localize the p2O2 binding domain in E2F-1 and test whether p2O2 would inhibit the DNA-binding of E2F. (3) To examine the growth- regulatory activities of p202. For this purpose, we plan to localize the growth-inhibitory domain in p202. The proposed studies will contribute to our understanding of the cell growth-inhibitory activities of the IFNs.